This invention relates to a process for forming an organic thin film, and more particularly to a process for forming a light and radiation-sensitive resist film.
Heretofore, a wet process and a dry process are known as the process for forming an organic thin film, among which the wet process has a problem of solvent toxicity or a problem of solvent drying. Organic materials having a low solubility in a solvent or having no appropriate solvent such as polyacetal, etc. cannot be made into a thin film by the wet process. Furthermore, it is very difficult to form a thin film free from pin holes by the wet process.
On the other hand, the dry process so far proposed includes a plasma polymerization process [Japanese Patent Application Kokai (Laid-open) No. 53-12057], a process for sputtering an organic compound [Japanese Patent Applications Kokai (Laid-open) Nos. 58-7703 and 57-116771]. The dry process has an advantage of forming a thinner film with no pin holes, as compared with the wet process. However, in these dry processes, organic compound molecules are considerably damaged by electron impingement. Thus, the formed thin films are in a non-stoichiometric amorphous state, that is, in an amorphous state with indefinite structural units, and can hardly retain the chemical structure of starting material. The sputtering process generally has a disadvantage of low film-forming rate. Furthermore, when polymers are formed on a substrate by sputtering or plasma polymerization of organic compounds, three-dimensionally cross-linked polymers are liable to be formed, so that no intended photosensitivity can be obtained in the application to the resist, etc.
On the other band, a vacuum vapor deposition process using a laser beam is known. For example, a process for vapor depositing an aluminum nitride thin film in vacuum with laser heating [Japanese Patent Application Kokai (Laid-open) No. 51-141800], a process for vapor depositing a boron nitride thin film in vacuum with laser heating [Japanese Patent Application Kokai (Laid-open) No. 54-141799], a process for producing a diamond-form carbon film (Japanese Patent Application Kokai (Laid-open) No. 56-22616), etc. are known. These processes are vacuum vapor deposition processes based on laser heating, using a high power laser of relatively long wavelength such as CO.sub.2 laser or YAG laser as a heat source, and have such problems when applied to the formation of an organic thin film as thermal decomposition of organic materials, or only vaporization, resulting in a failure to form an effective thin film, or the film formed being an assembly of low molecules weight modecules with a low mechanical strength.
In the production of semi-conductor devices, a photoetching process is now known as a technique of forming a desired pattern on a semi-conductor substrate. To form a fine pattern on a semi-conductor substrate by the photoetching technique, it is necessary to make a resist film as thin as possible or to increase the resolvability when exposed to light or radiation.
It has been so far tried to produce the semi-conductor devices not by a wet process, but entirely by a dry process, but two steps, that is, the resist film-forming step and the development step, must have been carried out by a wet process. That is, in the resist film-forming step, said plasma polymerization process, a gaseous phase photopolymerization process [Japanese Patent Application Kokai (Laid-open) No. 53-120529], or said vacuum vapor deposition process with laser heating has been proposed as the dry process. The plasma polymerization process can form a thin, uniform organic film free from pin holes, but the organic thin film is liable to undergo three-dimensional cross-linking when polymerized under plasma irradiation, or to have an irregular chemical structure, or their functional groups sensitive to light or radiation are liable to be damaged. Thus, it is very difficult to form a resist film sensitive to light or radiation by the plasma polymerization process. The gaseous phase photopolymerization process can form a resist film capable of forming a fine pattern without any considerable damage to the chemical structure, but has a very slow film-forming rate, and thus is not much practical. The vacuum vapor deposition process with laser heating so far proposed uses a high power laser of relatively long wavelength as a heat source and has the problems as mentioned above, with a failure to produce an effective resist film.
On the other hand, in the step of forming a light or radiation-sensitive resist film by the wet process so far known, many pin holes are formed, if the thickness of the film is made smaller, as mentioned above, and it is difficult to uniformly form a satisfactorily thin resist film free from the pin holes, and thus the resolvability cannot be improved satisfactorily.
Resist materials having a very high sensitivity to X rays or electron beam and an excellent resolvability have been now developed for the wet process, but a large number of such resist materials cannot be duly evaluated owing to poor solubilities in solvents, though they have expectable distinguished resist characteristics.
Furthermore, the formation of a resist film by the wet process has said hard-to-solve problems, such as solvent toxicity, solvent drying, etc.